Diseases are often characterized by their unique molecular and/or genetic fingerprints. However, for many diseases, including cancer, this has yielded limited success; partly because there are many possible ways the molecular pathways in a cell may become pathological, there is much to learn.
Cancer is still a leading killer in the United States, despite decades of focused research activity on the problem. However cells, aside from being biochemical and genetic entities, are also mechanical entities which have physical properties such as elasticity. Diseases which effect the cytoskeletal protein network of cells (i.e. the structural integrity of the cell), including cancer, should naturally yield cells with altered mechanical properties (e.g. elasticity). This area of research is in still in its infancy, but recent research has successfully been able to distinguish cancerous cells from normal ones based on experimentally measuring an effective cellular elasticity (see ref. Guck et al.) by optical means.
Existing methods of applying forces to cells show that cancerous cells have increased deformability, as has been described in Guck, J., et al., “Optical Deformability as an Inherent Cell Marker for Testing Malignant Transformation and Metastatic Competence”, Biophysical Journal 88:3689-98, 2005. It is known that photons, which carry momentum, are capable of exerting net forces on the center of mass of microscopic objects that have a refractive index different than the medium's (e.g., optical tweezers). Experiments done previously have shown that the surface forces that are generated can be much higher than the net forces, and are strong enough to deform biologically important objects, such as cells (see Guck, J., et al., “The Optical Stretcher: A Novel Laser Tool to Micromanipulate Cells”, Biophysical Journal 81:767-84, (2001)). The Optical Stretcher of Guck et al. shows that among cells which otherwise look identical, the cancerous cells show significantly greater deformability when optical forces are applied.
Specifically, in the Optical Stretcher, two counter-propagating beams trap and stretch cells in suspension, one at a time. The magnitude of the stretching is recorded on a microscope mounted CCD camera and a measurement of the aspect ratio of the stretched cell (major axis over minor axis) yields an optical deformability parameter. The deformabilities measured are actually a convolution of the mechanical and optical properties (i.e., refractive index) of the cell, and so elastic moduli have to be inferred from these measurements based on knowledge of the optical properties, unlike the direct mechanical probe of a scanning force microscope (see Lekka, M., et al., “Elasticity of normal and cancerous human bladder cells studied by scanning force microscopy”, European Biophysics Journal 28:312-6 (1999)). This is not a significant obstacle since for clinical purposes and most scientific purposes, statistical significance and robustness in a deformability measurement parameter is sufficient, and a rigorous calculation of the elastic moduli is not required.
Guck et al. have shown that the measured deformability correlates well with invasiveness, and that the optical deformability measure is related to the aspect ratios of the cells (major axis over minor axis).
The real limitation of the Optical Stretcher technique is, however, that only suspended cells may be measured. Unfortunately this is a non-physiological state for cells to be in, since they normally make cell-cell contacts and have a solid extracellular support (i.e., solid tissue). Since cells that do not have any surface to adhere to quickly lose their focal adhesion points and related stress fibers, there may be significant changes in the elastic properties of cells, compared to their in vivo properties, that are artifacts of the suspension conditions.
In other techniques to measure the elastic properties of cells, scanning force microscopy has been successfully applied to compare normal and cancerous cell elasticity on a surface, but this method suffers from two main difficulties: 1) the technique is exceeding slow (only several cells may be measured per day) making it difficult to envision translating into the clinical world, and 2) it is difficult to avoid mechanical contact with the sample, and so probe needle contamination is a real danger when measuring a series of cells.
Accordingly, a technique which can measure cells on a surface, which avoids damage to the cell, and which can speed up the technique to make it commercially viable, is needed.